LA mansionization rules

How big the house can get in LA

In LA's single-family zones the house, the garage and every other building on the lot share one floor-area limit, set as a share of the lot. The Baseline Mansionization Ordinance covers flat lots; the Baseline Hillside Ordinance covers lots in a Hillside Area.

LAMC 12.03, 12.07–12.08 C, 12.21.1 · Hillside: 12.21 C.10

In force. Numbers read from the LAMC current through 30 June 2026.

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The rule

Floor area is capped at a share of the lot area. In R1 the cap is 45%.
R1 height: 28 ft with a flat roof, 33 ft with a roof sloped 25% or more.
In R1, above 20 ft the walls step in at 45° from the front and side yard lines.
On a hillside lot the cap depends on the slope. Without a slope survey you get a guaranteed minimum.

The numbers

45%of the lot, R128 / 33 ftflat or sloped roof, R120 ftwhere R1 walls start to step in5 ftR1 side yards, lots 50 ft or wider15 ftR1 rear yard20 ftR1 front yard at most

Flat lots, by zone
Zone	Floor area cap	Height, flat / sloped roof
R1	45% of the lot	28 / 33 ft
RS	45% under 9,000 sq ft; 40% above, at least 4,050 sq ft	28 / 33 ft
RE9	40% under 15,000 sq ft; 35% above, at least 6,000 sq ft	28 / 33 ft
RE11	40% under 15,000 sq ft; 35% above, at least 6,000 sq ft	30 / 36 ft
RE15, RE20, RE40	35% of the lot	30 / 36 ft
RA	25% under 20,000 sq ft; 20% above, at least 5,000 sq ft	30 / 36 ft

RA, RE and RS get 20% more floor area if the upper floor is no bigger than 75% of the ground floor, or the front wall steps back. The R1 front yard is 20% of the lot depth, up to 20 ft; lots under 50 ft wide use side yards of 10% of the width, at least 3 ft.

What you can build

Add on, one floor or two, up to the cap. A plain addition needs plan check only, no hearing.
Build an ADU on top of the cap. State law allows an ADU of at least 800 sq ft whatever the floor-area limit (Gov. Code 66321(b)(3)).
On a hillside lot, add up to 500 sq ft past the cap if the house was permitted before 1 August 2010 (LAMC 12.21 C.10(l)).

What it doesn’t allow

Add on if the house is already at or over the cap (LAMC 12.23 A.1(c)).
Keep an old yard line for the new part. An addition must meet today's yards, even if the house sits closer now (LAMC 12.23 A.1(a)).
Leave the garage out of the count. A garage past its 200–400 sq ft allowance, covered porches and other buildings all count.
Use these numbers in an R1 variation zone, a specific plan, an HPOZ, the Coastal Zone or under Q or T conditions. Those set their own limits.

Hillside lots

Guaranteed without a slope survey: 25% of the lot in R1, 23% in RS, 20% in RE9 and RE11, 18% in RE15 to RE40, 13% in RA, or 800 sq ft if that's more. A slope band analysis can show more.
An addition can require bringing the parking up to a two-car garage (LAMC 12.21 C.10(g)).

Sample reportFrom a real report, address hidden

An RE9 lot in the San Fernando Valley

Lot: 9,686 sq ft. Cap: 40% = 3,874 sq ft.
House on record: 2,090 sq ft.
Room to add: about 1,784 sq ft, before counting the garage and porches.
Height 33 ft with a sloped roof. Yards: 25 ft front, 6.5 ft sides, 25 ft rear.

The official text

A plain-English summary, checked against the code text in November 2026. It isn’t legal advice; the code text controls.

Your lot · $49.99

What does LA mansionization rules mean for your lot?

The lot report works out your zone's cap from the lot area, takes off the house on record, and gives the room left, the height and the yards. Where the County's building outline and the Assessor disagree, it says so.

See what this means for your lot